PURPOSE: To test the relationship between cell killing and non-repaired DNA strand breaks both in repair proficient and deficient cell lines. MATERIALS AND METHODS: Five of the cell lines used are repair competent (CHO, CHO K1, rat rhabdomyosarcoma R1H, mouse balb and normal human fibroblasts), while four display a reduced repair capacity (scid, xrs1, xrs5, AT). Cell survival was determined by colony formation assay. The total number of strand breaks was measured by the alkaline unwinding technique and the numbers of double-strand breaks by constant-field gel electrophoresis. RESULTS: The nine cell lines showed a broad spectrum in radiosensitivity with SF2 values ranging from 0.018 to 0.58. The cell lines did not vary in the number of induced strand breaks, neither for all strand breaks nor for double-strand breaks alone. In contrast, there was a large variation in the number of non-repaired strand breaks measured 24 h after irradiation. Comparison of cell killing with the number of non-repaired breaks measured after a dose of 90 Gy showed no correlation for single-strand breaks (r2=0.29) but a fairly good correlation for double-strand breaks (r2=0.87). This correlation was found to hold both for repair proficient and deficient cell lines. CONCLUSIONS: The results obtained strongly suggest that the number of non-repaired double-strand breaks measured 24h after irradiation can be used as an indicator of cellular radiosensitivity.
